This Center would expand on-going, coordinated, multi-disciplinary studies of the immunology, virology, medicine and clinical epidemiology of chronic fatigue syndrome (CFS). The Clinical Cores would support the resources needed to follow over 1200 patients already cared for and enrolled in studies by the Pl and Co-Pl. From these patients, detailed clinical, laboratory and psychologic evaluations already have been obtained in a standardized format, and entered into a computerized data base. A large on-going bank of frozen serum, plasma and cells would be expanded. The databases and frozen specimens will be used by the Center investigators, and made available to other investigators, to test new hypotheses and observations about CFS. Three projects would be initiated by the Center. The first would examine the specificity of several laboratory abnormalities that have been reported in CFS -- hematologic, serum chemistry, immunologic and virologic -- by comparing patients with CFS to patients with fibromyalgia, major depression and healthy control subjects. The second project would employ a sensitive and accurate quantitative polymerase chain reaction (PCR) technique to evaluate possible overproduction or underproduction of various cytokines, and one cytokine receptor, in patients with CFS. The third project would evaluate the point prevalence of CFS -- an affliction sometimes thought to be restricted to middle- and upper-class Caucasians -- in a variety of minority populations.